The present invention relates generally to error detection systems for detecting errors of discretely-encoded signals, and, more particularly, to a bad frame indicator for detecting bad frames of information signals received by a receiver constructed to receive discretely-encoded signals comprised of coded frames.
A communication system operative to transmit information includes, at minimum, a transmitter and a receiver interconnected by a transmission channel. A radio communication system is a communication system in which the transmission channel is comprised of a radio-frequency channel. A transmitter which transmits an information signal upon the radio-frequency channel must convert the information signal into a form which may be transmitted upon the radio-frequency channel. The process by which the information signal is converted into a form which may be transmitted upon a radio-frequency channel is referred to as modulation. In a modulation process, the information signal is impressed upon a radio-frequency electromagnetic wave. The characteristic frequency of the radio-frequency electromagnetic wave is of a value which corresponds in frequency to be within a range of frequencies defining the radio-frequency channel. The radio-frequency electromagnetic wave is commonly referred to as a carrier wave, and the carrier wave, once modulated by the information signal, is referred to as a modulated, information signal. The modulated, information signal occupies a frequency bandwidth comprising a range of frequencies centered at, or close to, the frequency of the carrier wave. The modulated, information signal may be transmitted through free space upon the radio-frequency channel thereby to transmit the information signal between the transmitter and the receiver.
Various techniques have been developed for modulating the information signal upon the carrier wave. Such techniques include amplitude modulation (AM), frequency modulation (FM), phase modulation (PM), and complex modulation (CM). A receiver receives the modulated, information signal transmitted upon the radio-frequency channel, and contains circuitry to detect, or recreate otherwise, the information signal from the modulated, information signal transmitted thereto. This process is referred to as demodulation. Typically, the receiver contains both demodulation circuitry for demodulating the received signal, and, additionally, down conversion circuitry for converting downward in frequency the radio-frequency, modulated, information signal.
Numerous transmitters may be operative simultaneously to modulate and transmit information signals over different radio-frequency channels. As long as the signals transmitted by the numerous transmitters are transmitted upon different radio-frequency channels, no overlapping of simultaneously-transmitted signals occur. Receivers positioned to receive the transmitted signals contained tuning circuitry to pass only signals transmitted upon a desired radio-frequency channel.
The electromagnetic frequency spectrum is divided into frequency bands, each of which defines a range of frequencies of the electromagnetic frequency spectrum. The frequency bands are further divided into channels, such channels being referred to hereinabove as radio-frequency channels. Such channels are also frequently referred to as transmission channels. To minimize interference between simultaneously-transmitted signals, transmission of signals upon the channels of certain ones of the frequency bands of the electromagnetic frequency spectrum is regulated.
For instance, in the United States, a portion of a 100 MHz frequency band, extending between 800 MHz and 900 MHz, is allocated for a radiotelephone communication. Portions of corresponding frequency bands are similarly allocated for radiotelephone communications in other geographical areas. Radiotelephone communication may, for example, be effectuated by radiotelephones utilized in a cellular, communication system. Such radiotelephones include circuitry to permit both reception and transmission of modulated, information signals.
A cellular, communication system is formed by the positioning of numerous base stations at spaced-apart locations throughout a geographical area. Each base station contains circuitry to receive modulated, information signals transmitted by radiotelephones, and circuitry to transmit modulated, information signals to the radiotelephones.
Careful selection of the positions in which each of the base stations is located permits at least one base station to be within the transmission range of a radiotelephone positioned at any location throughout the geographical area. Portions of the geographical area proximate to individual ones of the base stations are defined to be associated with the individual ones of the base stations, and a base station and the portion of the geographical area associated therewith are defined to be a "cell". A plurality of cells, each associated with a base station, together form the geographical area encompassed by the cellular, communication system. A radiotelephone positioned within the boundaries of any of the cells of the cellular, communication system may transmit, and receive, modulated, information signals to, and from, at least one base station.
Increased usage of cellular, communication systems has resulted, in many instances, in the full utilization of every transmission channel of the frequency band allocated for cellular, radiotelephone communication. As a result, various ideas have been proposed to utilize more efficiently the frequency band allocated for radiotelephone communications. More efficient utilization of the frequency band allocated for radiotelephone communications increases the transmission capacity of a cellular, communication system.
One such means by which the transmission capacity of the cellular, communication system may be increased is to utilize a digital, or other discrete, modulation technique. When an information signal is converted into discrete form, a single transmission channel may be utilized to transmit, sequentially, more than one information signal. Because more than one information signal may be transmitted upon a single transmission channel, the transmission capacity of an existing frequency band may be increased by a multiple of two or more.
Typically, an information signal is first converted into discrete form (such as, for example, by an analog-to-digital converter), and then encoded by some coding technique prior to modulation and transmission thereof over a transmission channel.
Coding of the signal increases the redundancy of the signal, and such redundancy facilitates accurate determination of the signal once received by a receiver. A radio-frequency channel is not, however, a noise-free transmission channel; therefore, noise and other transmission difficluties, may cause a receiver to receive a signal other than that which was transmitted by the transmitter. Because an encoded signal contains redundancies, the receiver oftentimes may accurately decode the received signal to determine the actual information signal even when the encoded signal has been distorted during transmission thereof. Various block coding and convolutional coding techniques have been developed to facilitate accurate recreation of an information signal. One such convolutional coding technique is a Viterbi coding technique.
When distortion of the transmitted signal results in the receiver receiving bursts of distorted information, the decoder incorrectly decodes the received signal. Such incorrect decoding of the received signal results in the receiver recreating a signal other than the intended, information signal.
Parity bits oftentimes are included as a portion of the encoded signal transmitted by the transmitter. When a receiver receives the encoded signal having parity bits of values which are different than a predetermined sequence of values, that portion of the signal is ignored by the receiver. However, by random process, the parity bits may be of values indicative of an undistorted signal, and a receiver may incorrectly determine that a distorted signal has been accurately transmitted, and recreate thereby an incorrect signal.
For instance, when a discrete, encoded signal is comprised of sequences of digitally-encoded words (also referred to as frames), parity bits may be interspersed among, or concatenated to the bits which comprise the word or frame. If three parity bits are transmitted with each word or frame, the parity bits may form any of eight combinations. While a receiver must detect a specific combination of values of the parity bits to indicate that a valid signal has been received by the receiver, by random process, an undesired signal, such as a noise-only signal, may have values corresponding to the desired combination of parity bits. When a noise-only signal is received by the receiver, and the receiver searches for three parity bits per word or frame, the receiver may incorrectly determine that an invalid signal is a valid word as often as one out of eight times.
When a base station and a radiotelephone communicate in a process referred to as discontinuous transmission (DTX), the base station and radiotelephone transmit information only when information is detected at the radiotelephone. At all other times, the transmitter portion of the radiotelephone is inoperative to conserve radiotelephone power, while the receiver portion of the radiotelephone remains operative to detect reception of valid information. However, when the base station does not transmit information to the radiotelephone (referred to as non-transmit periods), the receiver portion of the radiotelephone receives only noise.
Because, by random process, a noise-only signal may be interpreted by a receiver as valid information one out of eight times when the receiver searches for the values of three parity bits, the receiver incorrectly determines that a noise signal is valid information signal one out of eight times. At a word or frame rate of 217 Hz, a noise-only signal may be incorrectly determined to be a valid information signal by the receiver 27 times per second. Such incorrect determination by the receiver results in undesired noise levels (sometimes audibly noticeable as squelching to be processed by the receiver).
What is needed, therefore, is a more accurate system by which invalid signals may be rejected by a receiver.